The actions and template data in a JSP page is written using the
JspWriter object that is referenced by the implicit variable out which
is initialized automatically using methods in the PageContext object.

This abstract class emulates some of the functionality found in the
java.io.BufferedWriter and java.io.PrintWriter classes,
however it differs in that it throws java.io.IOException from the print
methods while PrintWriter does not.

Buffering

The initial JspWriter object is associated with the PrintWriter object
of the ServletResponse in a way that depends on whether the page is or
is not buffered. If the page is not buffered, output written to this
JspWriter object will be written through to the PrintWriter directly,
which will be created if necessary by invoking the getWriter() method
on the response object. But if the page is buffered, the PrintWriter
object will not be created until the buffer is flushed and
operations like setContentType() are legal. Since this flexibility
simplifies programming substantially, buffering is the default for JSP
pages.

Buffering raises the issue of what to do when the buffer is
exceeded. Two approaches can be taken:

Exceeding the buffer is not a fatal error; when the buffer is
exceeded, just flush the output.

Exceeding the buffer is a fatal error; when the buffer is exceeded,
raise an exception.

Both approaches are valid, and thus both are supported in the JSP
technology. The behavior of a page is controlled by the autoFlush
attribute, which defaults to true. In general, JSP pages that need to
be sure that correct and complete data has been sent to their client
may want to set autoFlush to false, with a typical case being that
where the client is an application itself. On the other hand, JSP
pages that send data that is meaningful even when partially
constructed may want to set autoFlush to true; such as when the
data is sent for immediate display through a browser. Each application
will need to consider their specific needs.

An alternative considered was to make the buffer size unbounded; but,
this had the disadvantage that runaway computations would consume an
unbounded amount of resources.

The "out" implicit variable of a JSP implementation class is of this type.
If the page directive selects autoflush="true" then all the I/O operations
on this class shall automatically flush the contents of the buffer if an
overflow condition would result if the current operation were performed
without a flush. If autoflush="false" then all the I/O operations on this
class shall throw an IOException if performing the current operation would
result in a buffer overflow condition.

print

Print a string. If the argument is null then the string
"null" is printed. Otherwise, the string's characters are
written to the JspWriter's buffer or, if no buffer is used, directly
to the underlying writer.

println

Print an Object and then terminate the line. This method behaves as
though it invokes print(Object) and then
println().

Parameters:

x - the Object to write

Throws:

java.io.IOException - If an error occured while writing

clear

public abstract void clear()
throws java.io.IOException

Clear the contents of the buffer. If the buffer has been already
been flushed then the clear operation shall throw an IOException
to signal the fact that some data has already been irrevocably
written to the client response stream.

Throws:

java.io.IOException - If an I/O error occurs

clearBuffer

public abstract void clearBuffer()
throws java.io.IOException

Clears the current contents of the buffer. Unlike clear(), this
method will not throw an IOException if the buffer has already been
flushed. It merely clears the current content of the buffer and
returns.

Throws:

java.io.IOException - If an I/O error occurs

flush

public abstract void flush()
throws java.io.IOException

Flush the stream. If the stream has saved any characters from the
various write() methods in a buffer, write them immediately to their
intended destination. Then, if that destination is another character or
byte stream, flush it. Thus one flush() invocation will flush all the
buffers in a chain of Writers and OutputStreams.

The method may be invoked indirectly if the buffer size is exceeded.

Once a stream has been closed,
further write() or flush() invocations will cause an IOException to be
thrown.

Specified by:

flush in interface java.io.Flushable

Specified by:

flush in class java.io.Writer

Throws:

java.io.IOException - If an I/O error occurs

close

public abstract void close()
throws java.io.IOException

Close the stream, flushing it first.

This method needs not be invoked explicitly for the initial JspWriter
as the code generated by the JSP container will automatically
include a call to close().

Closing a previously-closed stream, unlike flush(), has no effect.

Specified by:

close in interface java.io.Closeable

Specified by:

close in class java.io.Writer

Throws:

java.io.IOException - If an I/O error occurs

getBufferSize

public int getBufferSize()

This method returns the size of the buffer used by the JspWriter.

Returns:

the size of the buffer in bytes, or 0 is unbuffered.

getRemaining

public abstract int getRemaining()

This method returns the number of unused bytes in the buffer.

Returns:

the number of bytes unused in the buffer

isAutoFlush

public boolean isAutoFlush()

This method indicates whether the JspWriter is autoFlushing.

Returns:

if this JspWriter is auto flushing or throwing IOExceptions
on buffer overflow conditions